Pneumatically actuated liquid dispensing valves generally utilize a liquid dispensing portion including a liquid inlet for receiving liquid such as hot melt adhesive, and a liquid outlet for discharging the liquid in various forms such as beads, dots, swirled filaments or spray forms. The dispensing valve further includes a valve member, such as a valve stem or needle mounted for reciprocating movement between open and closed positions. An actuation portion includes a pneumatic actuator such as a piston coupled for movement with the valve stem. The piston resides in a piston chamber and a dynamic seal is positioned between the piston chamber and the liquid dispensing portion to prevent leakage of liquid into the piston chamber or leakage of pressurized air from the piston chamber into the liquid dispensing portion. The valve stem extends through the dynamic seal. Pressurized air is introduced into the piston chamber to move the piston and the attached valve stem from the open position to the closed position and/or vice versa. Conventional pneumatically actuated valves may be air-over-air type in which pressurized air is introduced into a lower portion of the piston chamber to raise the piston and valve stem while air is exhausted from the upper portion of the piston chamber, and pressurized air is introduced into the upper portion of the piston chamber to force the piston and valve stem downward as air is exhausted from the lower portion of the piston chamber. Alternatively, such valves may be of the spring return type in which pressurized air is used to open the valve and a spring is used to close the valve. The upward and downward movements of the valve stem may open or close the valve depending on the design of the valve stem and the valve seat.
The dynamic seal associated with present dispensing valves results in friction and wear. This can lead to eventual leakage and the need to replace the seal or the entire valve. The friction between the dynamic seal and the valve stem can slow the response of the valve stem movement to the applied pneumatic pressure. Another possibility with current pneumatic dispensing valves is bleed over of pneumatic pressure from one side of the piston to the other. This can also adversely affect the performance of the valve.
It would be desirable to provide a liquid dispensing valve and method for pneumatically operating such a valve that addresses issues associated with using one or more dynamic seals.